Menthols, for their inherent mint flavor and refreshing feeling, have been widely used for medicines, foodstuffs, liquids, tablets, creams and pastes for oral introduction, etc. Also, these compounds are known to possess minor pharmacological utility as an antipruritic, mild local anesthetic, and counterirritant, according to the Merck Index, 8th Edition. However, since menthols are only slightly soluble in water, their use has heretofore been limited to (a) mixing in the form of solid particles directly with the material to which the menthol is to be added, (b) making a suspension using an emulsifier, or (c) dissolving the menthol in an organic solvent such as alcohol, etc. Therefore, the use of menthols has been confined and thus a menthol substance having good solubility in water has been much desired.
However, though menthols in the free form have an inherent mint flavor and certain pharmacological activities as noted above, they lose this flavor and pharmacological activity when converted into derivatives by reaction with other compounds. For this reason, the synthesis of a water-soluble menthol compound has never been attempted or even conceived so far as known.
With an aim of developing a menthol compound having good solubility in water, extensive research has been carried out. As a result, it has been found and demonstrated for the first time that menthol glycosides have high solubility in water and, while they per se do not manifest the characteristic mint flavor of menthols, they release this mint flavor when they are resolved into menthol and sugar by hydrolysis with various carbohydrases or acids, etc., as indicated in the following chemical equation: ##STR2## wherein R represents a residue of a monosaccharide or oligosaccharide. Thus, the menthol glycosides of this invention can be widely used as a water-soluble menthol substance.
It is well known that enzymes of the carbohydrase type are widely distributed in nature, occurring for example, in human saliva in the form of maltase, lactase, emulsin, amylase, etc., and that these enzymes are capable of hydrolyzing to cleave the glycosidic bond of glycosides. Further, it is also well known that common acids have a similar hydrolyzing capacity on glycosides in general.
It is conceived that a glycoside, in which the menthol acts as an aglycone, can manifest the mint flavor of the aglycone without drastic loss in water solubility, following the action of the enzyme or acid in cleaving the glucoside bond to yield the aglycone and sugar.